The present invention relates to a method and a system for balancing one or more supercapacitors.
Supercapacitors are known in the art and are currently being developed as power sources in high-power applications such as engine starting, power top-up for hybrid vehicle motors, and uninterruptible power supplies. In applications like these, power supplies are needed that can be charged quickly and can perform a very large number of cycles, which is the case with supercapacitors, but not with traditional batteries.
Supercapacitors are capable of delivering very high specific powers over short time periods. The characteristic discharging (or charging) time of a supercapacitor is of the order of a few seconds to a few tens of seconds, during which time specific powers exceeding 1 kW/kg can be delivered. Individual supercapacitors have a capacitance from 1 F to approximately 3 500 F and a very low resistance, of less than 1 mxcexa9 for components with the highest capacity.
When charging supercapacitors, it is important not to exceed a maximum voltage at the terminals of the supercapacitor. Controlling the charging of a supercapacitor so that charging is stopped if the voltage at the terminals reaches a predetermined value is known in the art. If the voltage exceeds this predetermined value, aging of the supercapacitor is accelerated, which reduces its autonomy and power.
A supercapacitor module generally comprises a plurality of supercapacitors connected in series. The applications mentioned above generally require voltages exceeding a few tens of volts, or even a few hundreds of volts. In this case, at the end of charging the supercapacitor module, a spread of the characteristics of the supercapacitors relative to each other is observed, and this applies in particular to the voltage at the terminals of the supercapacitors. This is due to a spread of the intrinsic properties (series resistance and capacitance) of each supercapacitor in the module, to aging of the supercapacitors, and possibly to a temperature gradient within the module, due to its environment. This leads to different leakage currents for each of the supercapacitors of the module and therefore to different end of charging voltages for each of the supercapacitors.
This problem compromises correct operation of the supercapacitor module. Some supercapacitors of the module may reach voltages exceeding their nominal charging voltage, which degrades their characteristics and leads to premature aging. Thus the module as a whole cannot function correctly.
To solve this problem, the document EP-0 851 445 proposes connecting a bypass circuit in parallel with the terminals of each supercapacitor of a module comprising a plurality of supercapacitors, the bypass circuit comprising in series a resistance and a zener diode whose characteristic is such that the current bypassed in the latter increases strongly, in accordance with the characteristics of the components, from a value slightly lower than the nominal voltage of each of the supercapacitors.
The general principle of supercapacitor balancing consists in bypassing some or all of the supercapacitor charging current in order to equate the end of charging voltage to a predetermined value.
The solution proposed in the above document is not entirely satisfactory, however. As soon as the voltage reaches a critical threshold, which is less than the nominal end of charging voltage of the supercapacitor, until it is balanced, the current in the supercapacitor becomes equal to the current in the bypass circuit at the nominal voltage of the supercapacitor. Accordingly, to ensure a leakage current in the bypass circuit less than the intrinsic leakage current of the components, the bypass current is very low, for example of the order of 10 mA for a supercapacitor having an intrinsic leakage current of the order of 1 mA, although the current needed to complete the charging of the supercapacitor is high, of the order of several tens of amperes. It therefore takes a long time to complete the charging of the supercapacitor, although supercapacitors are often intended to be charged with fast charge-discharge cycles.
Because a high holding current is necessary to keep the supercapacitors charged, using bypass circuits with a high leakage current also represents a penalty in terms of energy consumption and additional thermal dissipation.
When the bypass circuit is open circuit, its high leakage current discharges the supercapacitor into the bypass circuit, causing the supercapacitor voltage to decrease quickly.
An object of the present invention is therefore to develop a supercapacitor balancing method and system that significantly improve the dynamics of the end of charging and produce homogeneous characteristics in terms of the voltage at the terminals of the supercapacitors.
The remainder of the description refers to balancing in relation to a supercapacitor module and a supercapacitor in isolation, in which case it is more a question of monitoring the charging voltage of the supercapacitor.
The present invention proposes a supercapacitor balancing method including bypassing the current flowing in said supercapacitor from a predetermined threshold voltage at the terminals of said supercapacitor, the bypass current being a function of the voltage at the terminals of said supercapacitor and increasing continuously as a function of said voltage. In the method according to the invention said bypass current increases between said threshold voltage, for which said bypass current constitutes a minimum bypass current, and a reference voltage, for which said bypass current constitutes a nominal bypass current, and the difference between said reference voltage and said threshold voltage is less than 200 mV and the ratio between said nominal bypass current and said minimum bypass current is greater than 100.
Accordingly, thanks to the invention, the bypass current can reach high values, which ensures a higher charging current during the end of charging period and therefore a reduction in the duration of the end of charging period, combined with a supercapacitor leakage current which is of the same order of magnitude as its intrinsic leakage current. This is possible thanks to the method according to the invention which, unlike the prior art method, uses a bypass circuit with a sufficiently steep slope of the current-voltage characteristic, i.e. one in which the difference between the bypass current and the leakage current is sufficiently high over the bypass current range to ensure both a high bypass current and a low leakage current.
The increase in the bypass current as a function of the voltage at the terminals of the supercapacitor is advantageously linear, so that the reduction in the charging current (and the increase in the bypass current) is progressive.
It is even more advantageous if the bypass current is limited as a function of the voltage at the terminals of the supercapacitor. This preserves the integrity of the system if the voltage at the terminals of the supercapacitor significantly exceeds the reference voltage of the bypass circuit.
The invention can use a MOSFET, a bipolar transistor or an IGBT, for example, in the linear power amplifier of the bypass circuit in parallel with the terminals of the supercapacitor. The wide variety of components of this type that is available means that a component having the required characteristics in terms of nominal bypass current and power dissipation can be selected.
What is more, associating one of these components with a regulation system fixes the minimum voltage from which the bypass current increases and the maximum voltage from which the bypass current is limited to a constant value, as well as the value of this constant maximum current, in order to adapt it to the characteristics of the supercapacitor (maximum voltage at the terminals at the end of charging, charging current).
An additional advantage of using a MOSFET or a bipolar transistor is its very low leakage current (less than 1 xcexcA).
In the simplest embodiment of the invention, the reference voltage is advantageously equal to the end of charging voltage of the supercapacitor.
In a more sophisticated embodiment, the threshold voltage can be equal to the end of charging voltage of the supercapacitor. This optimizes charging, using charging protocols described later.
Accordingly, one highly advantageous variant of the method according to the present invention further includes monitoring the charging current of the supercapacitor as a function of the voltage at the terminals of the supercapacitor by a voltage detector logic function adapted to change from an activated state to a deactivated state if the voltage at the terminals of the supercapacitor exceeds the reference voltage, with the result that the supercapacitor charging current is reduced, and then to return to the activated state if the voltage at the terminals of the supercapacitor falls below a minimum monitoring voltage.
Thanks to this control function, the supercapacitor is not overcharged and the charging current at low current is reduced, with the result that the overall charging time is optimized This makes it possible to use a charging protocol with two levels of current and in which the charging current can exceed the nominal bypass current, which means that the charging time can be optimized without damaging the supercapacitor
This control function is obtained by means of the voltage detector function, which delivers in the form of a hysteresis signal, for example.
Another highly advantageous variant of the method according to the present invention includes monitoring the charging current of the supercapacitor as a function of the bypass current by a current detector logic function adapted to change from an activated state to a deactivated state if the bypass current exceeds the nominal bypass current, with the result that the charging current of the supercapacitor is then reduced, and then to return to the activated state if the bypass current falls below a minimum monitoring current. This also makes it possible to use a charging protocol with multiple current levels, and in which the charging current with multiple plateaus can exceed the nominal bypass current, as a result of which the charging time can be optimized without damaging the supercapacitor.
In particular, this allows the charging current to fall in a linear manner, combined with maintaining some bypass current.
The control function is again obtained by means of the current detector function which delivers a signal in the form of a hysteresis signal, for example.
In accordance with the invention, voltage control and current control can be combined. This makes it possible not to remain in the deactivated state when monitoring the voltage, even if the bypass current is substantially zero.
Finally, the invention also relates to a system for implementing the above method, in particular including a MOSFET, a bipolar transistor or an IGBT in the bypass circuit.
A supercapacitor module according to the invention can comprise a plurality of supercapacitors connected in series and a respective bypass circuit connected in parallel with the terminals of each of the supercapacitors or a single bypass circuit connected in parallel with the terminals of the set of supercapacitors.
A set of such modules can be connected in series and/or in parallel with a single bypass circuit connected to the terminals of the set.